Autotaxin (ATX), also known as ectonucleotide pyrophosphatase/phosphodiesterase (ENPP2), is a secreted ectoenzyme known to possess lysophospholipase D activity (Umezu-Goto et al., 2002), and is responsible for producing the bioactive lipid mediator lysophosphatidic acid (LPA) by the hydrolysis of lysophosphatidylcholine (LPC) (Tokumura et al., 2002). LPA is highly implicated in the pathogenesis of a number of physio-pathological diseases, including cancer (Liu et al., 2009; Mills & Moolenaar, 2003), neuropathic pain (Inoue et al., 2004) and fibrosis (Tager et al., 2008). Following the production of LPA, the lipid binds to specific G protein-coupled receptors of which there are seven known isoforms (Noguchi et al., 2009). Binding of LPA activates multiple signalling pathways (Mills & Moolenaar, 2003) including cell migration (van Dijk et al., 1998), proliferation and survival (Brindley, 2004). Other cellular responses include smooth muscle contraction, apoptosis and platelet aggregation (Tigyi & Parrill, 2003).
ATX was originally identified as a cell motility-stimulating factor following isolation from human A2058 melanoma cells (Stracke et al., 1992). Subsequent work on the enzyme was focused towards its role as a motility factor due to its aberrant expression in many cancer types including breast and renal cancer (Stassar et al., 2001), Hodgkin's lymphoma (Baumforth et al., 2005), follicular lymphoma (Masuda et al., 2008), as well as fibrosis of the lung and kidney (Hama et al., 2004). Ten years following its discovery, ATX was characterised as a secreted lysophospholipase (lysoPLD) (Tokumura et al., 2002; Gesta et al., 2002). Since then ATX gene knockout mice have shown that the ATX-LPA signalling axis plays a vital role during embryonic development of the cardiovascular and neural system (Tanaka et al., 2006; van Meeteren et al., 2006), resulting in early embryonic lethality (Bachner et al., 1999).
ATX belongs to a family of proteins called nucleotide pyrophosphatase/phosphodiesterase (NPP), encoded for by the gene ENPP. The family consists of seven structurally related enzymes (ENPP 1-7) conserved within vertebrates which are numbered according to their discovery. They were originally defined by their ability to hydrolyse pyrophosphate or phosphodiester bonds of various nucleotides and nucleotides derivatives in vitro (Stefan et al., 1999; Goding et al., 1998; Gijsbers et al., 2001), though ENPP2 and choline phosphate esters (ENPP6 & 7) have specific activity for other extracellular non-nucleotide molecules. ENPP2 (ATX) is unique within the family as it is the only secreted protein, whereas other ENPP members are transmembrane proteins (Stefan et al., 2005).
Hence, there is a need for potent inhibitors of ATX.
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